Generally, a light-emitting diode (LED) is a pn junction semiconductor device designed to emit light when forward biased. The light can be one of several visible colors such as red, amber, yellow, or green, or it can be infrared and thus invisible. Electrically, an LED is similar to a conventional diode in that it has a relatively low forward voltage threshold. Once this threshold is exceeded, the junction has a low impedance and conducts current readily. This current is limited by an external circuit, usually a resistor. The amount of light emitted by an LED is typically proportional to the forward current over a broad range; thus, it is relatively easily controlled, either linearly or by pulsing.
LED devices are used in a wide variety of commercial systems and many of these systems have the LED devices coupling radiation into optical fibers. It is known in the prior art such as from the article entitled, "Coupling of Spherical-Surfaced LED and Spherical-Ended Fiber" by Osamu Hasegawa, Ryosuke Namazu, Masayuki Abe and Yoshikazu Toyama, J. Appl. Phys. Vol. 51 (1), January 1980, pp. 30-36 that the launching efficiency for radiation from an LED device having a spherical lens is generally superior to that using an LED device with no lens. This article does not disclose information relating to the fabrication of spherical lenses.
Integrated spherical lenses for LED devices have also been analyzed and studied experimentally. Reported data confirm that LED devices having spherical lenses are much better than flat LED devices for specific types of LED devices such as laser diodes and infrared light emitting diodes according to the article entitled "High-Radiance Surface-Emitting (In,Ga) (As,P)/InP IREDS" by J. Heinen and Ch. Lauterbach, Siemens Forsch.-8 Entwick-lungsber, 1982, Vol. 11. This article discloses general steps of fabricating a spherical lens on an LED device by first chemically etching a mesa extending above a wafer surface and thereafter, chemically etching the mesa to round out the edges and the top of the mesa to form a spherical lens. The article does not include any specific details for fabricating the lenses.
One of the authors, Heinen, subsequently published a letter entitled, "Preparation and Properties of Monolithically Integrated Lenses on InGaAs/InP Light-Emitting Diodes", Electronic Letters, Vol. 3, pp. 331-332, 1982. Heinen discloses fabricating a lens by first chemically etching a circular mesa in the substrate centered to the emitting area. A conventional masking of a circular masked area was used. Heinen used bromine:methanol (1:40) as the chemical etchant. The etch mask used to form the mesa was removed and the same etchant was used on the mesa to round it into a spherical lens. The spherical lens extended above the surface of the LED device.
It is known in the prior art that an integrated spherical lens is preferably well formed to focus radiation optimally. Deviations from a true spherical lens can produce losses in the transfer of radiation from an LED device to an optical fiber or other components in an optical system.
Generally, a well shaped spherical lens can be etched from a well shaped mesa using an isotropic chemical etchant. A well shaped mesa has circular cross sections which are concentric and also has a top generally parallel to the surface of the LED device. Generally, an "isotropic chemical etchant" as used herein etches a flat surface of a specific material equally in all directions.
It has been recognized in the prior art that the fabrication of mesas having circular cross sections can be achieved with an isotropic chemical etchant for the wafers used to fabricate the LED devices. Generally, the lenses are fabricated in the portion of the wafer composed of InP. The mesas extend above the surface of the wafer in each case.
Some chemical etchants suitable for etching InP are disclosed in the article entitled, "Chemical Etching Characteristics of (001) InP", by Sadao Adachi and Hitoshi Kawaguchi, J. Electrochem. Soc., June 1981, pp. 1342-1349. These chemical etchants include (1) HCl systems, (2) HCl:HNO.sub.3 systems, (3) HBr systems, (4) H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 :H.sub.2 O systems, and Br.sub.2 :CH.sub.3 OH system. The article discloses the capabilities of these etchants to etch isotropically for applications such as etching mesas extending above the surface of a wafer.
One of the authors, S. Adachi, subsequently published an article entitled "Chemical Etching of InP and InGaAsP/InP", J. Electrochem. Soc., March 1982, pp. 609-613. In this article, an etchant composition of aqueous HBr, CH.sub.3 COOH(H.sub.3 PO.sub.4) and K.sub.2 Cr.sub.2 O.sub.7 is disclosed as being an improved chemical etchant for isotropically etching InGaAsP/InP wafers and having the additional advantage of not eroding photoresists such as commercially available AZ-1350. The article discloses that the ratio of the components can vary. The chemical etchant of the article is referred to in the article and herein as "BPK".
Adachi discloses that the etching rate of BPK is greater for a stirred solution than a non-stirred solution. Stirring does not produce a sufficiently random movement of the BPK to avoid non-uniform etching. Non-stirred solutions are preferable for etching. Adachi shows that a non-stirred BPK solution can etch a relatively pit free surface for concentrations greater than a certain level. Furthermore, for relatively low concentrations, the BPK solution tends to produce pits. Adachi concludes that the BPK is a good isotropic chemical etchant for both InP and InGaAsP/InP double heterostructure wafers and can be used to produce mesa-shaped structures with good resist pattern definition. Adachi does not, however, consider the suitability of the BPK for fabricating an integrated lens on an LED device.
An integral lens on an LED device can also be fabricated using photoelectrochemical etching as disclosed in the article entitled, "Photoelectrochemical Etching of Integral Lenses on InGaAsP/InP Light-Emitting Diodes", by F. W. Ostermayer, Jr., P. A. Kohl and R. H. Burton, Appl. Phys. Lett., Vol. 43, No. 7, October 1983, pp. 642-644. The invention disclosed herein relates to the use of a chemical etchant. The article is of interest because it discloses etched lenses which are below the surface of the LED device. There is no disclosure in the article relative to the advantages of such a configuration.
The following is a discussion of prior art commercial practices for fabricating integral spherical lenses on LED devices and the results of a study of these processes by the applicant.
The commercial fabrication of LED devices is typically carried out using a wafer as large as about 2 cm. by 2 cm. with a plurality of LED devices being formed in the wafer. Some of the difficulties in fabricating integrated lenses include chemically etching uniform mesas heights across the wafer and thereafter, etching smooth, well-defined spherical lenses from the mesas.
Tests have been carried out by the applicant using BPK and it has been found that good integrated spherical lenses can be fabricated when the lenses extend above the surface of the wafer. For such lenses, the mesas are produced by chemically etching the wafer surface around etch masks defining the top surfaces of the mesas. The etch masks are removed, and thereafter, the mesas are etched into spherical lenses.
A study by the applicant of the commercial process for fabricating raised spherical lenses on LED devices shows that the yields are not high due to many problems. One of the problems in the prior art is that the large surface area of the wafer being etched results in the chemical etchant near the edges of the wafer having a greater concentration than the chemical etchant near the center of the wafer. This is due to the chemical etchant near the edges being replenished more readily than the portion near the center of the wafer. As a result, the mesas near the edges of the wafer tend to be thinner than the centrally located mesas and the spherical lenses etched near the edges of the wafer are relatively small and undesirable.
Another problem in the prior art is that spherical lenses extend above the surface of a wafer and can be damaged relatively easily during subsequent processing steps. It has now been concluded that damages to lenses can be minimized by fabricating integrated spherical lenses below the surface of the wafer. Furthermore, the spherical lenses can be fabricated so that much less of the surface of the wafer is etched as compared to the prior art process.
An attempt was carried out by the applicant to fabricate integrated recessed spherical lenses on a wafer including LED devices using prior methods. Such recessed lenses were fabricated by initially chemically etching mesas defined by annular mask openings centered at the emitting areas of each LED device. The annular mask openings were defined by two concentric circles. It was planned that subsequently the masking would be removed and the mesas would be etched into the spherical lenses.
Surprisingly, it was found that mesas etched through the annular masks did not exhibit circular cross sections even though BPK was used. The BPK etched anisotropically when used on the annular mask openings. An analyst suggested that for this chemical etching, the mesas generally had slanted walls along the (111A) planes. This problem did not occur for mesas chemically etched to be above the surface of the LED device.
The chemical etching of mesas through annular openings was repeated with the change of making the annular openings much larger. Again, BPK was used. It was found that enlarging the annular openings can reduce or avoid the anisotropic etching because the chemical etching is closer to the conditions for which the mesas are formed above the surface of the wafer. This solution to the problem of anisotropic etching has serious drawbacks including leaving little space on each LED device for attaching a ball bond. Enlarging the area for each LED device reduces the number of devices which can be fabricated on a wafer.
It would be a commercial advantage to have a method of fabricating recessed spherical lenses on LED devices in which the lenses were well formed to provide good coupling of radiation, and in which the production yields of the LED devices were high.